At approximately day 1

Thymus arises from proliferations of the 3rd pharyngeal pouch. Thymic cells migrate to the anterior mediastinum, carrying parathyroid tissue, and finally fuse at the larynx.

Duration: 1 week

Reference [PubMed Link]

Sadler TW. 2000. Susceptible periods during embryogenesis of the heart and endocrine glands. Environ Health Perspect 108 (suppl 3):555-561.

At approximately day 1

Hematopoietic stem cells found in the liver.

Reference

Williams CB. 2003. Development of the immune system. In Rudolph’s Pediatrics 21st Edition, Rudolph CD & Rudolph AM, Editors. New York: McGraw-Hill.